Water evaporation, process gas heating, steam cracking, and pyrolysis of hydrocarbons are examples of processes often carried out in tubular coils, the process coils, inside furnaces. These processes are often considered heart of the industrial plant and have significant influence on the economics of the overall industrial plant. The duration that heater or tubular coils can operate without failure depends on two primary factors: fouling and crack initiation. Fouling and cracking are forms of coil degradation. Fouling occurs when deposits, such as coke and scale, build up on the inside surfaces of heating coil. These deposits in process fluid stream act as a resistance to heat flux and the outside metal temperature of the tube increases in response to allow for the equivalent flux through a higher resistance. The second factor is crack initiation, which depends strongly upon the makeup of the radiant heating coil, thermal stresses, and fatigues. Typically, the coil is made up of metal or metal alloy and has a nominal operating temperature range of from 400 K to 1400 K. Metals and metal alloys are sensitive to extreme temperatures. The coil will begin to deteriorate and become damaged or at least prone to damage when the coil is exposed to a temperature that exceeds the upper end of its nominal operating temperature range. As a result, a typical heater must be monitored carefully at substantial cost to maintain specific temperature ranges. This becomes problematic as deposits build up on the coil because more heat must be added to maintain the efficiency of the system.
For example, in a typical process gas heater used in the refineries or steel industries, the reformed gas, e.g. CO, H2, CO2, etc., mixture is preheated to a temperature of from about 400° C. to 600° C. This preheat occurs in the convection section of the heater. The mix then passes to the radiant section where a constant outlet temperature on the order of about 700° C. to 950° C. is maintained. The flue gas temperature exiting the radiant section of the fired heater is typically above 1,000° C. The heat transfer to the coils is primarily by radiation. In some conventional designs, such as boilers in power plants, approximately from 30% to 40% of the heat fired as fuel into the furnace is transferred into the coils in the radiant section. The balance of the heat is recovered in the convection section either as feed preheat or to superheat steam. Given the limitation of small tube volume to achieve short residence times and the high temperatures of the process, heat transfer into the reaction tube is difficult. As a result, high heat fluxes are used and the operating tube metal temperatures are close to the mechanical limits for even exotic metallurgies.
In most cases, tube metal temperatures limit the extent to which residence time can be reduced. A combination of higher process temperatures required at the coil outlet and the reduced tube length, i.e. the reduced tube surface area, results in higher flux and higher tube metal temperatures. Tube metal temperatures are also a limiting factor in determining the capacity of these radiant coils since more flux is required for a given tube when operated at higher capacity. The exotic metal reaction tubes located in the radiant section of the cracking heater represent a substantial portion of the cost of the heater. Therefore, it is important that they are operated at as high and as uniform a heat flux as possible consistent with the design objectives of the heater. This will minimize the number and length of the tubes and the resulting total metal surface area required for a given design capacity. Furthermore, having uniform heat flux across tube bundles will cause uniform thermal elongation of coils resulting in greater life of spring hangers on which coils are suspended and thus minimizes maintenance requirement.
In a typical furnace, the heat is supplied by burners, which can be mounted at the furnace floor, the furnace roof, the furnace sidewalls, or some combination thereof. The coils are typically suspended from the top of the radiant section and hang between the radiant walls. A small portion of the heat transferred is done convectively by the flue gases within the firebox transferring the heat directly to the coils. However in a typical furnace, greater than 85% of the heat is transferred by radiation.
In any flame from a burner, the flame has an inherent characteristic combustion profile, inherently generates heat, and inherently generates soot. As the fuel and air mixture leaves the burner, combustion begins. As the combustion reaction continues, the temperature of the combustion mixture increases and heat is released. At some distance from the burner, there is a point of inherent maximum combustion by the flame and hence an inherent maximum or peak heat release. During this process, heat is absorbed by the process coils. The characteristics of the flame, and its inherent maximum or peak heat, depend upon the total firing from that burner and the specifics of the burner design. Different flame shapes and heat release profiles are possible, depending upon how the fuel and air are mixed. Because of the characteristic heat release profile from these burners, an uneven heat flux profile, i.e. heat absorbed profile, is sometimes created. The typical flux profile for the radiant coil shows a peak flux near the center elevation of the firebox, i.e. at the point of maximum combustion or heat release for the hearth burners, with the top and bottom portions of the coil receiving less flux. In some heaters, radiant wall burners are installed in the top part of the sidewalls to equalize the heat flux profile in the top portion of the coil.
There have been a number of attempts to control the flux profile within a heater. It is known that staging the fuel to burners can be used to adjust the flame shape and thus impact the point of maximum heat release. Sometimes burners are designed with several differing fuel injection points. In some methods, side burners are used in combination with floor burners in a box chamber where combustion gases pass upwardly through the radiant chamber to a convention section. Methods of producing internal recirculation of combustion gases into the burner for producing a favorable influence on homogenizing combustible mixtures at the burners for reducing flame temperature and NOx emission have also been proposed. Still other methods have been proposed that depend on injection of steam in the furnace to reduce peak temperature and NOx emission. The results of these and other efforts, however, have not been entirely satisfactory, thereby necessitating further improvement in the art.